The regulation of immunoglobulin production has been analyzed by studying immunoglobulin expression in mouse myeloma hybrids and by isolating and characterizing cells with mutations in synthesis, assembly, and secretion. Many mouse myeloma X mouse myeloma hybrids were examined and all showed co-dominant expression of immunoglobulin. It was concluded that soluble repressors or activators were not important in regulating the expression of immunoglobulin genes. Secondly, a mutant cell line with a constant region deletion was studied in detail because it was also defective in glycosylation of the heavy chain. It was concluded that CHO was not necessary for synthesis, assembly, or secretion of the immunoglobulin. BIBLIOGRAPHIC REFERENCES: Regulation of immunoglobulin expression in mouse myeloma cells. Margulies, D.H., Cieplinski, W., Dharmgrongartama, B., Gefter, M.L., Morrison, S.L., Kelly, T. and Scharff, M.D. Cold Spring Harbor Symposia on Quantitative Biology, Volume XLI, 1977. A simple method for polyethylene glycol-promoted hybridization of mouse myeloma cells. Gefter, M.L., Margulies, D.H., and Scharff, M.D. Somatic Cell Genetics 3:231-236, 1977.